Color cathode ray tube deflection yoke evaluation

ABSTRACT

A method for evaluating the deflection center error between horizontal and vertical deflection coils in a deflection yoke for use with a shadowmask color cathode ray tube. The yoke to be evaluated is supplied with the usual vertical and horizontal deflection currents, and is placed in position on an operating cathode ray tube. The blue and green guns of this tube are turned off to establish a red raster, and an axial magnetic field is applied to the tube at the faceplate. Field intensity is adjusted to produce a color pattern having a red bullseye surrounded by alternating areas or sectors of blue and green, with small red color areas in the corners of the screen. The purity magnets are adjusted to center the bullseye. While maintaining a constant axial location, the yoke is rotated by at least a quarter turn. As the yoke is rotated any video patterns present will appear to rotate but the color pattern will remain essentially fixed. Any error in deflection centers between the horizontal and vertical coils will be manifested by motion in the blue and green color areas and the red corner spots. A yoke with no error in deflection centers will produce no motion of the color pattern during rotation.

Unite States Patent 91 Aher July 17, 1973 COLOR CATHODE RAY TUBEDEFLECTION YOKE EVALUATION [75] Inventor: Leon J. Aber, Lafayette Hill,Pa.

[73] Assignee: Phllco-Ford Corporation,

Philadelphia, Pa.

[22] Filed: Apr. 17, 1972 [2]] Appl. No.: 244,724

Primary Examiner-Carl D. Quarforth Assistant Examiner-J. M. PotenzaAttorney-Ro be rt D. Sanborn andGail W. Woodward roll! [57] ABSTRACT Amethod for evaluating the deflection center error between horizontal andvertical deflection coils in a deflection yoke for use with a shadowmaskcolor cathode ray tube. The yoke to be evaluated is supplied with theusual vertical and horizontal deflection currents, and is placed inposition on an operating cathode ray tube. The blue and green guns ofthis tube are turned off to establish a red raster, and an axialmagnetic field is applied to the tube at the faceplate. Field intensityis ad justed to produce a color pattern having a red bullseye surroundedby alternating areas or sectors of blue and green, with small red colorareas in the corners of the screen. The purity magnets are adjusted tocenter the bullseye. While maintaining a constant axial location, theyoke is rotated by at least a quarter turn. As the yoke is rotated anyvideo patterns present will appear to rotate but the color pattern willremain essentially fixed. Any error in deflection centers between thehorizontal and vertical coils will be manifested by motion in the blueand green color areas and the red corner spots. A yoke with no error indeflection centers will produce no motion of the color pattern duringrotation.

6 Claims, 2 Drawing Figures Patehted July 17', 1973 3,746,

COLOR CATI-IODE RAY T-UBE DEFLIECTION YOKE EVALUATION BACKGROUND OF THEINVENTION In a color cathode ray tube (CRT) of the shadowmask variety,as is currently used in the majority of color television receivers,three separate electron guns excite three separate phosphors on thefaceplate. The phosphors are broken up into small dots arranged intriads of triangular form. Each triad includes a red, blue, and greenclot and is aligned with a mating hole in the shadowmask so that eachcolor dot is energized by the correct electron gun. The relationshipbetween the shadow-mask and the phosphor dots is such that when anelectron beam emanates from the correct point in the neck of the tube,its angle of arrival at the shadowmask will be such as to projectthrough the hole in the shadowmask and fall on the correct phosphor dot.This same condition will prevail for all deflection angles. Thisgeometrical condition will be satisfied only if the electron beams areoriented to pass along the axis of the tube and if their center ofdeflection is made to coincide with that center for which the tube wasdesigned.

Color purity is the term used to describe the ability of an electron gunto activate only phosphor dots of the intended color. In a color CRTdisplay two adjustments are ordinarily applied to achieve color purityover the scanning raster. The first adjustment, for electron beamcentering, is provided by a purity magnet assembly that ordinarilymounts just ahead of the CRT socket. This device provides a magneticfield normal to the tube axis and adjustable in magnitude and direction.This field compensates for CRT assembly errors and is adjusted so thatthe undeflected electron beams fall in the center of the faceplate. Thesecond adjustment is the provision for moving the CRT deflection yokealong the neck of the tube in an axial direction. When the actual centerof electron beam deflection coincides with the tube design center, thedeflected beams will also fall on the correct phosphor dots.

Clearly, to achieve good color purity, the horizontal deflection coilmust produce a center of deflection that registers with that of thevertical deflection coil. Thus color CRT yokes have a further criticalassembly requirement not found in monochrome yokes. It has been foundthat for good color purity the deflection centers should be no more thanabout thirty thousandths of an inch apart. If the deflection centers donot coincide, the yoke cannot be adjusted to achieve color purity overthe entire raster. For example if the horizontal coil is aligned so thatits deflection center falls at the tube design center, correct beamlandings will be observed on that horizontal line that passes throughthe center of the faceplate. Any vertical deflection will result inincorrect beam landing and if the error is sufficiently great, anincorrect phosphor can be excited thereby contaminating the colorpurity. This effect will be more pronounced at greater deflectionangles. In practice the yoke position will be compromised to achieve thebest possible color purity over the entire raster. This will occur for ayoke position that places the tube design deflection center somewherebetween the actual deflection centers of the vertical and horizontalcoils.

The additional performance requirement imposed on color yokes hasproduced the problem of how to measure the coil deflection centers toevaluate yokes. This can be important in quality control in yokemanufacturing and at incoming component inspection in TV receivermanufacturing.

The standard measurement method involves connecting the yoke tobe'tested to a conventional display. Only one gun is operated during thetest. The system is first degaussed, the purity magnets adjusted forcorrect beam landing in the center of the display, and the yokepositioned for best color purity for deflected beams. Then a microscopicexamination of beam landing points is made. The center dot beam landingis centered on the phosphor dot by observing the dot through amicroscope and precisely adjusting the purity magnet so that theilluminated portion falls in the exact center of the dot. The verticaldeflection coil is evaluated by observing a dot a measured distanceabove (or below) the center dot and the lack of beam landingconcentricity measured. Knowing the CRT geometry, the location of thedeflection center can be computed. The same procedure is applied to thehorizontal deflection coil by observing the beam landing a measureddistance to the right (or left) from the center dot. It can be seen thatvery careful measurements must be taken accurately to compute thedeflection center. This can best be done by making the measurements onan enlarged calibrated photograph of the beam landing. Other measurementmethods are available, some employing special CRT structures andequipment. However the methods of measuring are practically the same.

The above methods, while acceptably accurate, are very time consumingand will only operate on CRT designs where the phosphor dots aresubstantially larger than the electron beams. In the recently developedblack matrix tubes, where the electron beams are larger than thephosphor dots, the above nethods cannot be used at all. The methods areclearly not amenable to mass production.

SUMMARY OF THE INVENTION It is an object of this invention to provide asimple means of indicating the degree of coincidence between deflectioncenters of the horizontal and vertical coils of a color CRT deflectionyoke.

It is a feature of the invention to-permit rapid evaluation of yokeswithout resorting to precision measurement.

It is a further feature to permit evaluation of yokes on CRTs of thetype having phosphor dots smaller than the electron beams.

These and other objects and features are accom plished by operating theyoke to be tested on a test jig comprising a color CRT display thatcould be a conventional color TV receiver. The display is operated withlittle or no video to produce flat background and two of the CRTelectron guns are turned off leaving a single color (usually red ispreferred). Al'll axial magnetic field of suitable magnitude is appliedat the CRT faceplate. The radial components of this field will have theeffect of rotating the scanning raster so that non axial electron beamswill be deflected in proportion to the offcenter distance. This producesa display having a central red bullseye surrounded by six wedge shapedcolor regions arranged in an alternating blue-green progression aroundthe bullseye. At the extremes of deflection the beam landing shift issufficient to pass beyond adjacent blue and green dots back into the reddot of the adjacent triads so that red corner spots are present.

At this point the purity magnets are set to center the red bullseye onthe faceplate, following which the yoke is pushed up against the CRTbell.

While maintaining a fixed axial position against the bell, the yoke isrotated on the neck of the CRT and the color pattern is observed. If theyoke has coincident horizontal and vertical deflection centers, theabovedescribed color display will remain unchanged during yoke rotation.(However video modulation patterns, if any, will rotate with the yoke.)Lack of deflection center coincidence will be indicated by motion of thecolor regions, particularly the red corner regions. Such motion will berelated to the difference in (or error of) the deflection centers.

This method can be applied to any known form of shadow-mask color CRT,including those having electron beams larger than the phosphor dots.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 shows in partial schematic-blockdiagram form the elements required for practicing the invention; and

FIG. 2 shows the pattern of the preferred display of the CRT used inFIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1 aconventional shadowmask color CRT is shown at 1. It will be located in acradle (not shown) as part of a test jig. It is preferred that the tubebe in the 19V to 25V size range for convenient observation but othersizes will serve. The deflection yoke to be evaluated is shown at 2.Aside from the purity magnet assembly at 3, mounted forward of socket 4,other CRT components, such as the blue lateral magnet, the convergenceassembly, and centering magnets are dispensed with. A power supply 5provides operating potentials for the CRT including high voltage, focus,and the electron gun voltages necessary to operate at least the red gun.Power supply 5 also powers two deflection sources, one at 6 forproducing a sawtooth of current at the horizontal frequency, and one at7 for producing a sawtooth of current at the vertical frequency. Thesewaveforms do not need to be synchronized and may be of onlyapproximately correct frequency.

All of the components described thus far are found in a conventionalcolor TV receiver or display monitor and these devices can be employedfor the yoke testing.

Coil 8, which could consist of 200 close wound turns of number 25 wire,may have an outside diameter of about 12 inches. This coil is secured tothe faceplate of CRT l, by means not shown, so that the coil axis isaligned with the axis of the CRT. Coil leads 9 connect to control box 10which contains a switch and rheostat. Switch 11 has three positions, thecenter position being the off position. The right hand position ofswitch 11 is the "degauss position. As the switch is thrown to theright, a damped wave generator 12 applies an alternating current to coil8 of diminishing value which, after a short time, decays to zero. Thisaction will degauss adjacent magnetic structures, including the CRTshadowmask and associated parts.

When switch 11 is thrown to the left or test position, power supply 5supplies direct current to coil 8. The magnitude of the current isadjusted by rheostat 13. Because of the coil location, the magneticfield produced by this current coincides with the CRT axis and theundeflected central electron beam but forms an increasing angle forbeams farther off axis. It can be seen that this field appears at theinterior of the CRT and will effectively rotate the electron beamscanning raster about the center of the CRT faceplate. The amount ofrotation is proportional to the field intensity and radial distance fromthe center of the CRT.

At the preferred field value, and with the deflection yoke located inits correct position, the display will appear as shown in FIG. 2. Withonly the red gun on, central region or bullseye 20 will be red. Regions21 are those where the magnetic field has shifted beam landings off ofthe red phosphor dots to the adjacent dots which will be blue or greendepending upon the polarity of the field. If regions 21 are blue,regions 22 will be green. Regions 23 are still farther from the axis andrepresent still more beam landing shift. These regions will be redbecause the dot in the triad adjacent blue or green is a red one.

The yoke test sequence will be as follows. First yoke 2 is located inapproximately its normal position on the CRT and the CRT is degaussed.Then, with the red gun on and switch 11 in the test position, rheostat13 is adjusted for the desired pattern. Purity assembly 3 is adjusted tocenter the red bullseye 20 on the CRT face. Yoke 2 is then pushed upagainst the CRT bell. This may cause the color field regions to assume asomewhat asymmetrical shape and the edges may not be straight. Howeverthe CRT bell provides a convenient means for holding the yoke in aconstant axial position and prevents yawing. The yoke is then rotatedabout the tube axis by at least The color pattern, particularly inregions 23, is observed during yoke rotation. Lack of motion of thecolor pattern indicates coincidence of the horizontal and verticaldeflection centers. Motion therefore is related to the error indeflection center coincidence.

If desired the yoke can be held in a mechanical device that permits theyoke to be rotated without yawing. This allows'the test to be made atthe desired yoke axial position and avoids distortion of the colorpattern. However this refinement is not necessary.

The above process can be performed quickly and the character of adefective yoke ascertained immediately. The test could be used to screenincoming yokes to a receiver production line and this could be done on alOO-percent or a lot-sampling basis.

If desired, the color pattern could be outlined on a particular tubewith grease pencil or crayon. If, upon yoke rotation, a particular yokeproduces more than a prescribed amount of motion, it is rejected. If themotion remaips within tolerance the yoke is accepted.

Calibrated markings can be achieved using test yokes of knowncharacteristics. For example a given test setup can be first calibratedwith a yoke having negligible deflection center error and the colorareas marked on the tube face. Then a yoke having a known error that isconsidered barely acceptable is connected to the test device. The motionof the color regions is then observed and marked with a second set ofoutlines defining the limits of acceptability. Such a test yoke can beobtained by employing the laborious prior art method of measurement. Theacceptable limit can be established by selecting a yoke where themicroscopic beam landing observation shows that the error in landing, if

increased further, will result in color purity degradation.

After the yoke test, switch 11 can be moved to the degauss position andthe damped wave train will remove any residual magnetic field that thetest might have produced.

While the above preferred embodiment teaches the invention, alternativeswill occur to a person skilled in the art. For example an adjustablepermanent magnet could be used in place of the coil. Also, thedegaussing function could be accomplished with a spinning permanentmagnet and operated by removing it while spinning from the vicinity ofthe CRT. While the coil 8 is shown as round other shapes could be used.It is intended that the scope of the invention be limited only by thefollowing claims:

I claim:

1. A process for evaluating the coincidence of deflection centers of thehorizontal and vertical deflection coils of a magnetic deflection yokefor use with a shadowmask color cathode ray tube; said process beingcarried out on an operating shadow-mask cathode ray tube displayincluding a shadowmask cathode ray tube having a neck portion and a bellportion, a power supply for producing the currents and potentials tooperate said tube, horizontal and vertical deflection current supplysources, and an externally mounted adjustable color purity magnetassembly; said process comprising:

a. mounting the yoke to be tested in approximate position on said neckportion of said tube,

b. connecting said yoke to said deflection current supply source,

c. operating said tube so that only one of the three electron guns insaid tube is functional,

(1. establishing a magnetic field having an axial direction in said bellportion of said tube,

e. adjusting the magnitude of said field to produce a color patternhaving a central bullseye surrounded by six, generally wedge-shapedalternating color regions,

f. adjusting said purity magnet assembly on said tube to produce abullseye color corresponding to the desired color for the energizedelectron gun and to center said bullseye on the screen of said tube,

g. rotating said yoke around the axis of said tube while maintaining afixed axial position to produce at least a rotation, and

h. observing the motion of said color pattern and the motion of thejuncture between said alternating color regions as said yoke is rotated,said motion being indicative of the degree of lack of coincidencebetween said horizontal and vertical deflection centers.

2. The process of claim 1 wherein said yoke is rejected as defectivewhen the observed motion of said color regions exceeds a predeterminedvalue.

3. The process of claim ll wherein the CRT is degaussed prior to theprocess steps recited in clause (d).

4. The process of claim 3 wherein the CRT is again degaussed followingthe yoke evaluation.

5. The process of claim 1 wherein the magnitude of said field isadjusted in the process step recited in clause (e) to produce a colorpattern further characterized as having corner color regions of thecolor of said bullseye, and the observing step identified in the processstep recited in clause (h) includes observing the motion of said cornercolor regions.

6. The process of claim 1 wherein said fixed axial position identifiedin the process step of clause (g) is maintained by pushing said yoke upagainst said bell portion of said tube.

1. A process for evaluating the coincidence of deflection centers of thehorizontal and vertical deflection coils of a magnetic deflection yokefor use with a shadowmask color cathode ray tube; said process beingcarried out on an operating shadowmask cathode ray tube displayincluding a shadowmask cathode ray tube having a neck portion and a bellportion, a power supply for producing the currents and potentials tooperate said tube, horizontal and vertical deflection current supplysources, and an externally mounted adjustable color purity magnetassembly; said process comprising: a. mounting the yoke to be tested inapproximate position on said neck portion of said tube, b. connectingsaid yoke to said deflection current supply source, c. operating saidtube so that only one of the three electron guns in said tube isfunctional, d. establishing a magnetic field having an axial directionin said bell portion of said tube, e. adjusting the magnitude of saidfield to produce a color pattern having a central bullseye surrounded bysix, generally wedge-shaped alternating color regions, f. adjusting saidpurity magnet assembly on said tube to produce a bullseye colorcorresponding to the desired color for the energized electron gun and tocenter said bullseye on the screen of said tube, g. rotating said yokearound the axis of said tube while maintaining a fixed axial position toproduce at least a 90* rotation, and h. observing the motion of saidcolor pattern and the motion of the juncture between said alternatingcolor regions as said yoke is rotated, said motion being indicative ofthe degree of lack of coincidence between said horizontal and verticaldeflection centers.
 2. The process of claim 1 wherein said yoke isrejected as defective when the observed motion of said color regionsexceeds a predetermined value.
 3. The process of claim 1 wherein the CRTis degaussed prior to the process steps recited in clause (d).
 4. Theprocess of claim 3 wherein the CRT is again degaussed following the yokeevaluation.
 5. The process of claim 1 wherein the magnitude of saidfield is adjusted in the process step recited in clause (e) to produce acolor pattern further characterized as having corner color regions ofthe color of said bullseye, and the observing step identified in theprocess step recited in clause (h) includes observing the motion of saidcorner color regions.
 6. The process of claim 1 wherein said fixed axialposition identified in the process step of clause (g) is maintained bypushing said yoke up against said bell portion of said tube.